Use lazyness for FastString's z-encoding memoization

Having an IORef in FastString to memoize the z-encoded version is
unecessary because there is this amazing thing Haskell can do natively,
it's called "lazyness" :)
We simply remove the UNPACK and strictness annotations from the constructor
field corresponding to the z-encoding, making it lazy, and store the
(pure) z-encoded string there.
The only complication here is 'hasZEncoding' which allows cheking if a
z-encoding was computed for a given string. Since this is only used for
compiler performance statistics though it's not actually necessary to have
the current per-string granularity.
Instead I add a global IORef counter to the FastStringTable and use
unsafePerformIO to increment the counter whenever a lazy z-encoding is
forced.